1. Technical Field
The invention relates to a method of driving a CCD type solid-state imaging device having a plurality of photoelectric conversion elements, which are arranged on a semiconductor substrate in a row direction and a column direction perpendicular to the row direction, and a charge transfer device that reads out electric charges generated in the plurality of photoelectric conversion elements and that transfers the read electric charges in the column direction, which is a charge transfer direction.
2. Description of the Related Art
Japanese Patent Nos. 2609363 and 3483261 disclose a technique for improving the sensitivity of a CCD type solid-state imaging device by multiplying electric charges at the time of charge transfer through avalanche multiplication in a charge transfer channel.
FIG. 11 is partially sectional views schematically illustrating a charge transfer device disclosed in Japanese Patent No. 2609363.
As shown in FIG. 11, a charge transfer device is configured to include a charge transfer channel 200 formed in a silicon substrate, an insulating layer 201 formed on the charge transfer channel 200, and a plurality of electrodes 202 to 205 formed on the insulating layer 201. As shown in FIG. 11(a), a driving pulse on which a reading pulse is superimposed is supplied to the electrode 203 so that a potential well (hereinafter, referred to as “packet”) P1 is formed in the charge transfer channel 200 and below the electrode 203 and that electric charges generated in a photoelectric conversion element (not shown) are read out to the packet P1. Then, as shown in FIG. 11(b), a pulse having a predetermined level is supplied to the electrode 204 to form a packet P2 in the charge transfer channel 200 and below the electrode 204. At this time, levels of the pulses supplied to the electrodes 203 and 204 is set so that a potential difference between the packet P1 and the packet P2 becomes a value required to cause the avalanche multiplication.
A difference between depths of the packets P1 and P2 occurs due to a level difference between the pulses supplied to the electrodes 203 and 204. The depth of the packet P2 is larger than that of the packet P1. Accordingly, electric charges existing in the packet P1 move to the packet P2. In this case, since the electric charges move through a high electric field region d generated between the electrodes 203 and 204, the avalanche effect occurs in the high electric field region so that the electric charges are multiplied. After multiplying the electric charges, a level of a pulse supplied to the electrode 204 is changed to a level that causes the packet P2 to be a barrier against the packet P1. Thus, the packet P2 returns to its original state as shown in FIG. 11(c). Then, the electric charges existing in the packet P1 are transferred in the charge transfer direction by supplying transfer pulses to the plurality of electrodes 202 to 205.
In the method of driving a charge transfer device described above, the following problem arises because there is a potential difference between barrier potentials at both ends of the packet P2 for causing the avalanche multiplication as shown in FIG. 11. In the case when the packet P2 is formed as shown in FIG. 11(b), even unnecessary charges (referred to as “unnecessary charges A”), such as noises generated on a surface of the charge transfer channel 200 below the electrode 205, move to the packet P2 together with the electric charges existing in the packet P1. In addition, the unnecessary charges A are also multiplied in the high electric field region between the electrodes 204 and 205. On the other hand, unnecessary charges (referred to as “unnecessary charges B”) exist even in the packet P1. Accordingly, the unnecessary charges B are also multiplied in the same manner as described above. Moreover, a region d2 between the electrodes 204 and 205 is larger in electric field than a region d1 between the electrodes 203 and 204. Therefore, the unnecessary charges A are larger in number than the unnecessary charges B. Also, the large electric field causes breakdown of free electrons in the region d2. Accordingly, a multiplication factor of the unnecessary charges A is larger than that of the unnecessary charges B. As a result, S/N is deteriorated.